The invention relates to a method for applying a coating to the surface of a thin rigid board or a similar structure. The invention is particularly useful for coating base materials in the production of circuit boards with fluid resist.
Usually the basic boards for printed circuit boards have a length of about 185 to 600 mm and a width of about 145 to 350 mm and are 0.2 to 4 mm thick. The basic boards comprise resin-bonded paper or synthetic resins reinforced by glass fibres or similar composite materials and are relative rigid. They are provided with a thin layer of copper on which a fluid photoresist lacquer is to be applied as a light sensitive coating. The coating is 0.004 or 0.008 mm thick and any thickness between these values and also thicker and thinner coatings may be required. The boards have one edge for handling which remains uncoated. This uncoated marginal zone should not be broader than 10 mm. If the boards are handled in horizontal position gripped only on the narrow marginal zone it can break off or the boards will bend thereby preventing uniform treatment of all parts of a surface. Very often the boards are to be coated on both sides so that they cannot be supported by rolls, guide bars or the like before the coating is sufficiently dry. Supporting the boards by air streams will result in different quality of the dried coatings on both surfaces. But is is already difficult to obtain coatings of identical thickness on both surfaces if the board is coated in horizontal position simultaneously or in two steps.